Terminal for heating furnaces



nited States Patent Ofice 2,809,223 Patented Oct. 8, 1957 TERMINAL FOR HEATING FURNACES Cary H. Stevenson, Chicago, Ill., assignor to Lindberg Engineering Company, Chicago, Ill., a corporation of !Ilinois Application September 26, 1955, Serial No. 536,407

8 Claims. (Cl. 13-25) This invention relates to terminals for heating furnaces and more particularly to an electrical terminal extending through a furnace wall for transmitting electricity to a heating element in the furnace.

In electric furnaces for heat treating, melting and the like, it has been customary to extend terminals through the furnace wall connected to a heating element in the furnace and to a bus bar or other supply means outside of the furnace. It is desirable to make such terminals as large as possible to minimize the IR drop through them, thereby to minimize heat losses in the furnace wall and outside of the furnace. On the other hand, large terminals carry a large amount of heat by conduction from the furnace interier and this consideration dictates the use of small terminals to minimize heat loss. The practical results of these conflicting demands has been a compromise to keep the heat losses due to IR drop in the terminal and to conducton through the terminal within permissible limits.

It is one of the objects of the present invention to provide a terminal for heating furnaces which is of adequate section to minimze the IR drop in the furnace walls and outside of the furnace and is sufiiciently small where exposed to the heated interier of the furnace to minimize conduction heat losses.

A further object is to provide a terminal which tapers from a relatively small section on the interior of the furnace to a large section at the furnace exterior.

According to one feature of the invention, the terminal may be either a flat strip flaring in width from the interior to the exterior of the furnace or may be of pyramidal shape flaring in both width and thickness from the interior to the exterior of the furnace.

The above and other objects and features of the in- Vention will be more readily apparent from the following description when read in connection With the accompanying drawing, in which:

Figure 1 is a horizontal section through a heat treating furnace equipped with terminals embodying the invention; i

Figure 2 is a partial section on the line 2-2 of Figure 1;

Figure 3 is a vertical section through a melting furnace equipped with terminals embodying the invention;

Figure 4 is a perspective view of the heating element and terminals of Figure 3;

Figure 5 is a partial section View showing an alternative form of terminal embodying the invention; and

Figura 6 is a modification of the invention.

The heat treating furnace shown in Figure 1 comprises a hollow, generally rectangular body 10 formed of refractory material and which may be enclosed within a metal outer casing 11. The body has an opening 12 at one end normally closed by a door 13 through which the furnace may be charged or unloaded.

The furnace is heated by heating elements 14 in the form of metal strips or sheets which may be constructed as more particularly described and claimed in the cepending application of George W. Holz et al., Serial No. 569549, filed March 5, 1956. The strips 14 extend across the back and along the sides of the furnace interior and are connected at their opposite ends to terminals 15 and 16. Bus bars 17 connect the terminals 15 to a source of supply 18 and the terminals 16 may lead directly to the source of supply, as shown. Heated gases within the furnace chamber may be circulated over the charge by a fan or blower 19.

In this type of furnace the terminals 15 and 16 may constitute flat integral continuations of the strips 14 and may be shaped as best seen in Figure 2. As shown in this figure, applied to one of the terminals 15', the terminal is formed by a relatively narrow portion 21 connected to the strip 14 and extending substantially to the inner surface of the furnace wall 10. From the inner surface of the furnace wall to its connection to the bus bar 17 the terminal has an outwardly flared section 22 increasing from the minimum width of the narrow portion 21 to a substantially wider portion on the exterior of the furnace. It will be understood that the terminals 16 are similarly shaped.

With this terminal construction a minimum area of the terminal is exposed to the heated interior of the furnace through the narrow section 21 so that loss of heat by conduction to the terminal is reduced to a minimum. Any heat generated in the narrow terminal portion 21 is dissipated within the furnace chamber and is useful heat. As the section of the terminal increases through the flared portion 22 thereof, the current density in the terminal decreases rapidly so that heat loss due to IR drop outside of the furnace chamber is reduced to a minimum. With the present terminal, therefore, both sources of heat loss are minimized to produce maximum Operating efiiciency without the necessity of design compromise.

Figures 3 and 4 illustrate the invention as applied to a pot type melting furnace including a body 25 of refractory material which is generally cup-shaped and open at its top. An annular heating element 26 is mounted within the cavity within the body substantially coextensive with the height of the cavity and extending completely around it. If desired, the heating element 26 could be made in the form of several axially displaced rings, as disclosed in the co-pending application of George W. Holz et al. referred to above, or could be embedded in the walls of the furnace body adjacent to the interior, as disclosed in the patent to Lang, No. 2,694,740. When the heating element is not so embedded, a separate pot 27 is inserted in the furnace within the heating element to receive the metal to be melted.

The heating element 26 is energized through terminals 28 embedded in and extending through the refractory walls of the furnace body. As shown, three terminals 23 are provided equally spaced around the periphery of the heating element to be energized by three-phase heating current. It will be understood, however, that more or less terminals could be provided and that the heating element could be split if desired, as shown in the Lang patent referred to above.

According to the present invention, the terminals 28 are of somewhat thicker section than the heating element 26 and are welded thereto at their inner ends. The terminals fiare in the form of a truncated pyramid from their point of connection to the heating element to the outside of the furnace, as best seen in Figure 4. Therefore, the terminals present a minimum section at their point of connection to the heating element to minimize heat loss due to conduction and have an increasing section to the outside of the furnace walls so that heat loss due to IR drop outside of the furnace is minimized.

Figure 5 illustrates an alternative embodiment of the invention and is illustrative of a feature which can be employed in any of the embodiments shown in Figures 1 to 4. As illustrated in Figure 5, a terminal extends through a refractory furnace wall 31 and comes to a heating element shown at 32. The terminal in this instance is made of uniform cross section throughout its length with an inner portion 33 Secured to the heating element 32 by Welding or the like and an outer portion 34- joined to the inner portion 33 within the furnace wall and preferably adjacent to the inner surface thereof and extending through the furnace wall for connection to a bus bar or the like. As shown, the inner and outer portions may be Secured by welding or the like along a line 35 lying within the furnace wall.

In this Construction, the inner portion 33 of the terminal is formed of a material having a relatively low electrical conductivity such, for example, as nickel, chromium steel of the type employed in the heating element 32. The outer portion 34 of the terminal is formed of a material having a higher electrical conductivity such, for example, as conventional low carbon steel. With a terminal of this type, the loss due to current flow through the outer portion 34 of the terminal Will he substantially less than the loss due to current flow through the inner portion 33 so that losses due to current flow are confined largely to the interior portion of the furnace, as in the embodiments of Figures 1 to 4. At the same time, the terminal can be made of minimum cross section so that losses due to heat conduction through the terminal are minimized.

Figure 6 illustrates a further alternative embodiment of the invention which is in eifect a combination of Figures 1 through 4 With the feature of Figure 5. As shown in Figure 6, a heating element 36 which may be similar to the tube heating element 26 of Figures 3 and 4 or the strip 14 of Pigures 1 and 2 as connected thereto by weldng or the like, a terminal including an inner portion 37 connected to the heating element and an outer portion 38 connected to the inner portion 37. The inner portion 37 is formed of a material similar to that of the heating element itself with a relatively low electrical conductivity and of relatively small section. The outer portion 38 is formed of a material having a higher electrical conductivity such as conventional low carbon steel and is tapered in width to provide an increasing cross section from the point of its connection to the inner portion 37 to its outer end. This terminal is mounted in the furnace in the same manner illustrated in Figure with the inner portion 37 extending into the furnace wall and the outer portion 38 projecting through the furnace wall for connection to a bus bar or the like.

While several embodiments of the invention have been shown and described herein, it will be understood that they are illustrative only and not to be taken as a definition of the scope of the invention, reference being had for this purpose to the appended claims.

What is claimed is:

1. In a heating furnace including a hollow body of refractory material and an electric heating element in the body, a terminal extending through a wall of the body and connected to the heating element, the terminal having a portion of relatively small cross section at its inner end where it is exposed to the interier of the body and increasing in section toward its outer end to have a relatively large cross section at the outer surface of the body wall.

2. In a heating furnace including a hollow body of refractory material and an electric heating element in the body, a terminal extending through a Wall of the body and connected to the heating element, the terminal tapering smoothly in cross section from a portion of relatively large cross section at the outer surface of the body wall to a portion of relatively small cross section at the inner surface of the body wall.

3. In a heating furnace including a hollow body of refractory material and an electric heating element in the body, a terminal extending through a wall of the body and connected to the heating element, the terminal having a generally conical shape with its smaller end connected to the heating element and its larger end projecting through the outer surface of the wall.

4. ln a heating furnace including a hollow body of refractory material and a heating element in the body in the form of a strip of conducting material wider than it is thick, a terminal extending through a wall of the body and at its inner end connected to the heating element and being of a cross section at least as large as the heating element, the terminal flaring from its inner end to have a larger cross section at its outer end.

5. In a heating furnace including a hollow body of refractory material and a heating element in the bodyin the form of a strip of conducting material Wider than it is tlick, a terminal extending through a wall of the body and at its inner end connected to the heating element and being of a cross section at least as large as the heating element, the terminal being in the form of a flat strip fiaring in width toward its outer end.

6. In a heating furnace including a hollow body of refractory material and an electric heating element in the body, a terminal extending through a wall of the body and connected at its inner end to the heating element, the terminal being formed with a portion of relatively low electrical conductivity at its inner end and a portion of higher electrical conductivity at its outer end extending through the Wall of the body.

7. in a heating furnace including a hollow body of refractory material and an electric heating element in the body, a terminal extending through a Wall of the body and connected at its inner end to the heating element, the terminal being of substantially uniform cross section throughout its length and having an inner portion of a material having a relatively low electrical conductivity at its inner end and a portion of a material having a higher electrical conductivity at its outer end extending through the wall.

8. In a heating furnace including a hollow body of refractory material and an electric heating element in the body, a terminal extending through a wall of the body and connected at its inner end to the heating element, the terminal having an inner portion of relatively small cross section formed of a material of relatively low electrical conductivity and the outer portion which extends through the wall being of larger cross section and being formed of a material of higher electrical conductivity.

References Cited in the file of this patent UNITED STATES PATENTS 994,744 Hall June 13, 1911 l,046,8l6 Lightfoot Dec. 10, 1912 1,093,512 Whitney Apr. 14, 1914 1,298,609 Wentworth Mar. 25, 1919 1,350,S76 Mayer et al Aug. 24, 1920 1,367,341 Abbott Feb. 1, 1921 1,733,530 Dowiatt Oct. 29, 1929 2,125,075 Mackenzie July 26, 1938 2,615,060 Marinace Oct. 21, 1952 2,694,74O Lang Nov. 16, 1954 2,735,881 Mann Feb. 21, 1956 

